Electrostatic printing method and apparatus using developer powder projection means



Nov. 30, 1965 s. M FARLANE ELECTROSTATIC PRINTING METHOD AND APPARATUSUSING DEVELOPER POWDER PROJECTION MEANS Filed Aug 6, 1962 BY 6%, 45/14JMM ATTORNEYS United States Patent 3,220,831 ELECTROSTATIC PRINTINGMETHOD AND APPARATUS USING DEVELOPER POWDER PROJECTION MEANS SamuelMcFarlane, Summit, N.J., assignor to Sun Chemical Corporation, New York,N.Y., a corporation of Delaware Filed Aug. 6, 1962, Ser. No. 214,949 4Claims. (Cl. 961) This invention relates, in general, to the field ofelectrophotography and has as an objective the improving of xerographicand electrostatic printing techniques to provide image transfer by aforce field rather than through mechanical pressure to yield uniformprinting on various materials and even corrugated surfaces and as acontinuous printing operation when desired.

In the usual process of xerography as disclosed, for example, in CarlsonPatent No. 2,297,691, issued October 6, 1952, a photoconductiveinsulating material provided with a conductive backing is given auniform electric charge over its surface and is then exposed to light orradiation through an image to form an electrostatic latent image on theinsulating layer. The electrostatic charges making up the image areformed due to the light exposure discharging photoconductive surfaceareas to conductive backing plate in accordance with the light intensityimpinging upon the incremental surface areas. Development of the imageis effected by bringing oppositely charged printing particles to theelectric charge of the pattern itself such that they are held thereonclectrostatically in the outline of the electrostatic latent image.Thereafter, the developed image is transferred to a suitable material towhich it may be fixed, if required. In other embodiments, theelectrostatic latent image is transferred from one non-conductivesurface to another before it is used for printing purposes.

Owing to the internal electrostatic lines of force established in thelatent image due to the charge distribution, the prior art systems haveexperienced great difficulties with the development step in attemptingto overcome concentrated charges, which result in the copies reproducedhaving hollow centers and emphasized edges. Usually, some form of adevelopment electrode is required. Such electrodes have at least asurface of a conductive material biased slightly or held at the samepotential as the plate or backing conductive member to establishelectrostatic fields outwardly of the image and away from its backingplate to minimize the distortion.

The present invention essentially avoids these problems by providing astrong development field with definite lines of force establishedthrough an air gap between at least the image and the transfer medium ormaterial to be printed. The problem is further minimized by avoidingintermediate transfers of the electrostatic latent image, therebyeliminating handling of the charged images which heretofore causedsmudging.

By using an electrostatic latent image, the present invention eliminatesstencils and hence permits continuous printing of repetitive patterns orimages of printed or pictorial matter. In addition to this advantage,the use of an air gap for image transfer avoids the disadvantages ofpressure transfer which requires uniform contact and the attendant useof wetting agents, resulting in more uniform printing without bleeding,even on a corrugated transfer medium.

More particularly, apparatus in accordance with the invention employs,in one embodiment thereof, an electrostatic latent image-fonning carrieror plate in the form of a surface producedby a vast plurality ofdiminutive needle-like or' rod projections, each tipped or coated on3,220,831 Patented Nov. 30, 1965 their extremities with alight-sensitive or photoconductive layer. The needles are all set in aconductive member or film which is adapted along with a plate or otherconductive film to form an electrostatic field therebetween. Thenon-conductive coating of the image carrier elements is uniformlycharged and a light image focused thereon to produce an electrostaticlatent image, defined by charges distributed along the coated tips ofthe needles or rods. The discharged areas of the carrier lose theircharges through the photoconductive insulator coating to their internalneedles.

The thus formed electrostatic image is suitably dusted with a pigment inthe form of a fine printing powder, building up fairly sizable depositsof printing material along the properly charged coated needle tips inaccordance with the image pattern thereon. The dusted image is thenplaced in or caused to enter a strong electrostatic field developed inan air gap, which space also receives the transfer medium. The printingparticles are projected across the air gap and then to the transfermedium being specifically fixed thereto, if necessary, to providepermanent copy. By properly arranging the field polarity relative to thecharge of the particles, the particles are actually repelled from theimage carrier and attracted by the oppositely disposed field boundingplate, but are intercepted in their flight by the intermediatelydisposed transfer medium.

The foregoing purpose may be carried out in a continuous manner or instep-by-step fashion for individual image reproductions In the former, apattern is printed on a fabric or a film of images is reproducedcontinuously. In the latter, images are reproduced intermittently as inoflice business machines or the like.

The described process is capable of being carried out by hand orpartially by machine to produce xerographic plates for use in otherarrangements.

One unusual feature presented by the air gap charge transfer arrangementpermitting printing on rough or corrugated paper is the facility tocreate an illusion of depth. Embossed or specially indented paper can beprinted in the indentations to present the appearance of a threedimensional scene.

With the foregoing in mind, it is an object of the invention to providea novel printing method and apparatus capable of continuous printingfrom projected images ,without pressure contact with the printingmaterial;

A further object is the provision of such an apparatus which minimizesthe mechanical elements and relies upon electrostatic transfer throughan air gap to the transfer medium without the necessity of wettingagents;

It is a still further object to provide an arrangement whereinelectrostatic latent image techniques are employed in a contact-free airgap utilizing an image carrier capable of very heavy printing;

Other objects and advantages of this invention will become apparent tothose skilled in the art in view of the following detailed descriptionwhen considered in light of the attached drawing in which:

FIG. 1 is a schematic view in perspective of an image carrier beinguniformly charged;

FIG. 2 is a schematic view of the same carrier being exposed to animage;

FIG. 3 is a further similar schematic view of the soexposed imagecarrier being powdered with inking material;

FIG. 4 is a view in side elevation showing the plate of FIGS. 1 through3 actually in use in the printing process;

FIGS. 4a and 4b show detailsof the image carrier or plate and the fieldof FIG. 4; and f I FIG. 5 is a perspective schematic of apparatuscapable of the automatic printing operation described herein.

Referring now to the drawing and particularly to FIGS. 1 through 3,there is shown a novel type xerographic plate 11 which is formed of abacking member 13 embedded with or otherwise holding a plurality ofconductive needle-like or rod-like projections 15, all desirablycoterminous to form an image receiving surface.

The needles 15 are best illustrated in FIG. 4a wherein it may beappreciated that they. are quite closely spaced, numbering some 250tothe square'inch distributed over the face of the backing member 13.Each tip is coated to a depth of A inch or so as shown at 17 by a thinlayer of photoconductive material, such as, zinc oxide in an organicresin binder, which is commercially available.

In FIG. 1 a fur or plush roller 23 is shown being rubbed lightly anduniformly over the surface. formed by the photoconductive tips 17. Thismay be done by hand or carried out through an oscillatory motiondeveloped at the .slot 25 through suitable mechanical connections to theroller handle or extension 27.

The so-charged coatings 17 of the plate 11 are now exposed to a lightsource, schematically illustrated at 31, through an image 33 to bereproduced. This may be carried out by hand or automatically, but in anyevent should be done in the dark or a semi-lighted area (as in the caseof the complete process) in order that the electrostatic latent imagepattern formed on the photoconductive tips 17 will not be affected.

Next the electrostatic latent image is developed, as

shown in FIG. 3, by dusting onto it a suitable pigment, such as a finepowdered ink of carbon black, dry dye or other commercially availablemarking material in fine powder form. The powder particles may be givena charge opposite to that of the electrostatic latent image areas to beprinted such that they will cling only to the printing areas to outlinethe image. The so-developed image is brought into an electrostatic fieldfor printing purposes, as is illustrated in FIG. 4. A potentialdifference is established between a conductive plate 43 and theconductor 13 through an air gap generally indicated between the plates43 and 13, into which space the transfer medium, such as the corrugatedpaper 45 is disposed. The field intensity is quite high relative to themagnitude of the electrostatic charges such that the marking powder isprojected across the air space and onto the corrugated paper 43 toreproduce the developed electrostatic image.

An approximately field distribution is shown in FIG. 4b to reveal howaccurate but heavy printing is available even on rough or corrugatedsurfaces. The electrostatic lines of force 51 tend to establish parallelpaths from the needles of plate 13 to the opposite conductor 41 when thefield is initiated. The charged powdered ink particles tend to followthe electrostatic lines 51 and impact and stick to the transfer medium43 in fine printing areas in accordance with the electrostatic charges.The provisions of the relatively large area photoconductive surfaces,presented by the depth of coating on the needle tips, permits theaccumulation of extra heavy charges of printing material on the tips 17to provide the heavy printing mentioned.

It should be mentioned that the plate 13 may be of conductive materialwholly or it may comprise, for example, a moldable insulator which iscoated with a conductive film in electrical contact with the needles 15.

In FIG. there is illustrated schematically suitable apparatus forcarrying out the aforedescribed printing process continuously or inintermittent fashion. An endless conveyor 61 is shown comprising aflexible conductive film or strip 63 corresponding to the backing plate13 previously described. The film is of suflicient rigidity as tosupport the tipped or coated needles 65 corresponding 'to the needles 15of FIG. 4a. The endless conveyor 61 is supported on driving sprockets.71 and 73 for endless rotation. A common drive is shown in the form ofthe motor 75 adapted for control in any suitable conventional manner. 1

In the right hand intermediate portion of FIG. 5, there is shown a furor plush roller 77 of the type described as roller 23 in connection withFIG. 1. The roller 77 is lightly disposed against the conveyor needles65 to establish a uniform charge on the photoconductive surface of thetips. Since the rotation of the conveyor 65 is in a counterclockwisedirection, the conveyor presents a uniformly charged photo-sensitivesurface to a light image of a projection system, generally indicated at81.

A film strip 83 is shown as an endless loop pattern where continuousprinting of fabric or the like is desired. However, it will beappreciated that the pattern 83 could as well consist of a regular filmstrip or documents of images to be reproduced. A slit exposure mechanismcomprising projection means in general, including lens means 95, animage slit 97 and projection slit 89 are provided so that the lensprojects through the image slit a focused image of the projection slitin conventional manner.

The pattern or original document or film 83 is passed across theprojection slit 89 at the desired rate of speed through the sprocketdrive 91 which is, of course, synchronized with the rate of travel ofthe conveyor 63. Latent electrostatic images are produced on thephotoconductive surfaces of the conveyor needles 65. These images arecomprised of charges distributed along the surfaces of thenon-conductive needle tip coatings such that effectively a relativelythick image charge can be formed. The apparatus of FIG. 5 is disposed ina substantially light-tight container (not illustrated) such that thecharge pattern will not be affected during the printing process. 'Adusting chamber 95, provided with a continuously rotating brush 97 foragitating the powder, is shown disposed adjacent to the upper reach ofthe conveyor 63. The suitably charged inking particles are attarcted tothe light exposed areas of the conveyor needles 65 to stick thereto,thereby developing-the electrostatic image. These particles may comprisepowdered ink of a thermosetting variety or other similarprintingmaterial as mentioned or commercially used.

The now dusted latent image enters an electrostatic field of arelatively high value compared to the latent image charges. This fieldis formed between the backing strip 63 (or its conductive film) and theneedles 65 of the conveyor 61 and a flat electrode 101, similar to theplate 43 of FIG. 4. A difference of potential (preferably directcurrent) is established by the voltage source E effective over lead tothe plate 101 and lead 104 to roller 106 which is in contact with a flatstrip portion of the conductive backing 63 for the conveyor 61. Theelectrical potential difference is of the correct polarity to projectthe image particles through the air space (as illustrated in FIG. 4) tothe flat conductor 101. However, these particles are intercepted intheir flight by the transfer medium, shown as the paper 107, moving insynchronism with the conveyor 61 through the supply roll drive 109 whichmay also be powered from common drive 75. The paper 107 is maintained incontact with or adjacent to the flat plate conductor and under tensionor in a taut condition by the rollers 111.

The driving mechanism for the printing medium 107 and the endlessconveyor 65 are synchronized such that the printing surface moves at thesame speed through the electrostatic field as the electric image. Thespacing in the air gap (of necessity illustrated as grossly exaggerated)is made quite small in order that a relatively small voltage differencemay establish .sufiicient electrostatic force lines for detail printing.An air gap of several hundredths of an inch between'the conveyor needles65 and the printing plate 101 requires a voltage difference of severalhundred volts (preferably direct curent) for proper printing. Factorsaffecting the optimum printing voltage are: width of the gap, humidityin the air gap,

type pigment and paper. The effective gap width may be varied as much as0.1" for voltage differences of 1500 volts for printing.

It is believed that ionization of this air space is not necessary butthat the charged powdered ink particles are simply projected through theair space to impact the printing surface, the latter naturally beingtaut to maintain a substantially parallel relationship with the tips ofthe conveyor needles. The parallel relationship is not essential toprinting by this method or aparatus but the spacing should be uniform topreclude arcing at a narrow point and the illustrated apparatus providessuch uniformity. The uniformity is not critical within the limits ofordinary corrugated paper provided that the voltage difference is setaccordingly.

The so-transferred image is then fixed, if necessary, as by the heatingcoils 103 (when thermosetting ink is used) located along the printingsurface path of travel. The paper then exits through a light-tightopening in the housing (not shown).

It may be appreciated that in the embodiment of FIG. 5 either continuousor intermittent operation may be employed, it being only necessary tocorrelate the mechanical drive mechanism after Well-known principles.The apparatus may function as an office reproducing machine by simplyusing a holder for the document to be reproduced in the proper locationfor focusing an image of the entire document on conveyor 61. In such anarrangement, the device is preferably intermittently operable, a buttonor the like being provided for each reproduction operation, includingexposure and printing. The electrostatic latent image is produced duringa dwell time, as is also the printed copy.

Thus, the operation of the conveyor is cycled to charge the conveyorimage receiving area; to arrest conveyor movement for a light flash ofthe image onto its charged surface; to start conveyor movement to powderand deliver the image to the field producing area; to establish thefield during this dwell time to print, and to move the thus printed copythrough the fixing station and out of the machine. Suitable intermittenttype drives are well known to achieve the foregoing described operationand are therefore not illustrated here.

While the present invention, as to its objects and advantages, has beendescribed in relation to certain specific embodiments herein, it isunderstood that it be intended that the invention be construed withinthe spirit and scope of the appended claims wherein what is claimed 1.The method of electrostatic printing comprising the steps of producing alatent electrostatic charge image on the photoconductive light-sensitivecoated tips of a plurality of electrically conductive spaced-apartneedles set in an electrically conductive backing member in adistribution of some 200 or more per square inch, wherein the needlesare substantially parallel and have their tips terminating in a commonsurface to define a charge image carrier; powdering the image with fine,granular marking material along the tips on the coating; and,introducing the so-powdered image into an electrostatic fieldestablished in the air to project the marking material along the linesof electrostatic force onto a transfer medium spaced from the image inthe field for printing the image on the transfer medium, saidelectrostatic field being characterized by a voltage gradient ofapproximately 15,000 volts per inch and the spacing between the transfermedium and the powdered image on the coated tips of said needles beingof the order of at least several hundredths of an inch.

2. In apparatus for strong development field electrostatic depositionprinting, the comhination of an image carrier in the form of anelectrically conductive backing member having approximately 200 or morespaced-apart substantially coterminous electrically conductive rod-likeprojections defining a common surface; an insulating photoconductivecoating on the tip of each to a depth of approximately for receivingelectrostatic charges; means for forming an electrostatic latent imageon the coatings of the projection tips carried by the image carrier;means supporting the carrier for endless rotation; means for powderingthe image with marking material; means for establishing an electrostaticfield having an air gap bounded on one side by the carrier; and meansfor introducing the image carrier with the powdered image into the airgap along with a transfer medium in spaced-apart relation whereby themarking material is projected through a portion of the air gap toreproduce the image on the transfer medium; said field beingcharacterized by a voltage gradient of approximately 15,000 volts perinch and said portion of the gap being of the order of at least severalhundredths of an inch.

3. In apparatus for electrostatic printing, the combination of anelectrically conductive supporting member supporting a plurality ofelectrically conductive spaced-apart rod-like projections extendingoutwardly thereof and each having on its outer end a layer oflight-sensitive coating thereon; said projections numbering in excess of100 per square inch and said coating covering approximately ,5 of theouter extremity of each projection; means for producing an electrostaticlatent image on said coatings; means for applying granular printingmaterial on the image; a planar conductive member disposed inspacedapart relation with the projection ends of the supporting member;means for introducing a transfer medium into said spacing; and, separateconnections to the supporting member and the planar conductive memberwhereat a potential difference may be applied to project the printingmaterial in the direction of the planar conductive member forinterception by the transfer medium to print said image; said potentialdifference being sufficient to attract the charged granular printingmaterial to the transfer medium across the spacing between the coatedprojections and the transfer medium.

4. In apparatus for electrostatic printing, the combination of aflexible strip-like conductive backing member supported for rotarymovement; a plurality of electrically conductive needle-like projectionsextending outwardly of the member in spaced-apart substantially parallelalignment; light-sensitive coatings along the outer tip-ends of theprojections; said projections numbering in excess of 100 per square inchand said coating covering at least of the outer extremities of eachprojection; means for producing an electrostatic latent image on saidcoatings while they are moving; means for applying granular printingmaterial on the image; a planar conductive member disposed inspaced-apart relation with the projections tipends of the backingmember; means applying a potential difference between the conductivebacking member and the planar member of sufiicient magnitude to projectthe printing material in the direction of the planar conductive memberbut onto the transfer medium to print said image.

References Cited by the Examiner UNITED STATES PATENTS 2,599,542 6/ 1952Carlson.

2,682,478 6/1954 Howse 96-1 2,692,948 10/ 1954 Lion.

2,820,716 1/1958 Harmon et al. 1.7 2,917,385 12/ 1959 Byrne 96-13,056,136 9/1962 Macgritf 96-1 FOREIGN PATENTS 745,500 2/ 1956 GreatBritain.

OTHER REFERENCES Nicoll, RCA Technical Notes, RCA TN No. 375, June 1960.

NORMAN G. TORCHIN, Primary Examiner.

1. THE METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OFPRODUCING A LATENT ELECTROSTATIC CHARGE IMAGE ON THE PHOTOCONDUCTIVELIGHT-SENSITIVE COATED TIPS OF A PLURALITY OF ELECTRICALLY CONDUCTIVESPACED-APART NEEDLES SET IN AN ELECTRICALLY CONDUCTIVE BACKING MEMBER INA DISTRIBUTION OF SOME 200 OR MORE PER SQUARE INCH, WHEREIN THE NEEDLESARE SUBSTANTIALLY PARALLEL AND HAVE THEIR TIPS TERMINATING IN A COMMONSURFACE TO DEFINE A CHARGE IMAGE CARRIER; POWDERING THE IMAGE WITH FINE,GRANULAR MARKING MATERIAL ALONG THE TIPS OF THE COATING; AND,INTRODUCING THE SO-POWDERED IMAGE INTO AN ELECTROSTATIC FIELDESTABLISHED IN THE AIR TO PROJECT THE MARKING MATERIAL ALONG THE LINESOF ELECTROSTATIC FORCE ONTO A TRANSFER MEDIUM SPACED FROM THE IMAGE INTHE FIELD FOR PRINTING THE IMAGE ON THE TRANSFER MEDIUM, SAIDELECTROSTATIC FIELD BEING CHARACTERIZED BY A VOLTAGE GRADIENT OFAPPROXIMATELY 15,000 VOLTS PER INCH AND THE SPACING BETWEEN THE TRANSFERMEDIUM AND THE POWDERED IMAGE ON THE COATED TIPS OF SAID NEEDLES BEINGOF THE ORDER OF AT LEAST SEVERAL HUNDREDTHS OF AN INCH.